CDKs, the cyclin regulatory subunits and their natural inhibitors, the CDK tumor suppressor proteins (CDKIs), are central to cell cycle regulation and their functions are commonly altered in tumor cells. Deregulation of CDK2 and CDK4 through inactivation of CDKIs such as p16INK4a, p21WAF1, p27KIP1, and p57KIP2 may override the G1 checkpoint and lead to transformation. CDKs interact with certain cell cycle substrates through the cyclin binding motif (CBM) and form a complex with the cyclin groove of the G1 and S phase cyclins, a surface binding site involving a protein-protein interaction. It has been shown that CDK isoform and substrate selective inhibition may be achieved through the use of peptides that block recruitment of both pRb and E2F and potently inhibit CDK2/CA kinase activity. Inhibition of CDKs though the cyclin provides an approach to obtain selectivity against other protein kinases and inhibit only the G1 and S phase CDKs as only these contain a functional cyclin binding groove. In particular, CDKs that regulate the RNA polymerase-II transcription cycle should be unaffected by cyclin groove inhibitory (CGI) compounds. Although it has been shown that cancer cells depend on the RNAPII cycle to express anti-apoptotic genes and that inhibition of transcriptional CDKs leads to potent anti-tumor agents, it is at the same time likely that this will lead to effects in normal cells and may be responsible for toxicities observed with current CDK inhibitors being clinically evaluated.
The cyclin binding motif represents a consensus of the cyclin groove binding sequences found in many cell cycle and tumor suppressor proteins. CGI peptides in transducible form have been shown to induce cell cycle arrest and selective apoptosis in tumor cells in vitro. These permeabilized peptides also act as anti-tumor agents in that when administered directly to a SVT2 mouse tumor model, significant tumor growth inhibition was obtained and histological analysis showed that tumors underwent apoptosis.
The ATP competitive CDK inhibitors developed to date are generally non specific against the single variants in the CDK family. It is believed that a major component of the anticancer activity of CDK inhibitors is through the transcriptional inhibition of CDK7 and 9. While it has been suggested that transcriptional CDK inhibition may be beneficial for cancer therapy, it is also probable that this will lead to significant toxicities. The most selective CDK inhibitor described to date is a CDK4, 6 selective compound, PD0332991 (selective vs. CDK2/protein kinases (CDK4 IC50, 0.011 μmol/L; Cdk6 IC50, 0.016 μmol/L, no activity against 36 other protein kinases) although it has apparently not been tested against the transcriptional CDKs. Regardless, this compound is a potent antiproliferative agent against retinoblastoma (Rb)-positive tumor cells and induces a G1 arrest, with concomitant reduction of phospho-Ser780/Ser795 on pRb. Oral administration to mice bearing the Colo-205 human colon carcinoma xenografts resulted in marked tumor regression suggesting that it has significant therapeutic potential and that targeting CDK4/cyclin D may be a viable strategy. In addition to cyclins A and E, the D-type cyclins also contain a functional cyclin groove and CDK4/cyclin D dependent kinase activities may therefore be blocked by cyclin groove inhibitors.
Further oncology target validation for selective inhibition of CDK4/cyclin D has been demonstrated using models of breast cancer and where it was shown that mice lacking Cyclin D are highly resistant to mammary carcinomas induced by erbB-2 oncogene. Further research into the role of Cyclin D in tumor formation made use of a mutant form which binds to CDK4/6 but cannot promote catalytic activity. This kinase-defective Cyclin D/CDK complex results in more evidence of resistance to erbB-2 induced tumorigenesis in mice. Combination of these two studies strongly indicates that Cyclin D1/CDK4 kinase activity is required for erbB-2-driven tumorigenesis and therefore confirms that Cyclin D1/CDK4 is a promising oncology target. While there are several reports of potent and selective inhibitors of the CDK2/cyclin A, E substrate recruitment, with both peptidic and peptidomimetic compounds being identified, very little has been reported with respect to either inhibitors or on the requirements for binding to the cyclin groove of CDK4,6/cyclin D1.